Declare DllMain internally

DllMain is a symbol that needs to be global, but no one needs to know.
However, some compilers will warn if there isn't a declaration before
the function is defined.  Just add a declaration before the function
definition.
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

crypto/cryptlib.c

@@ -210,6 +210,7 @@ void OPENSSL_cpuid_setup(void)
  * detaches
  */
 
+BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved);
 BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
 {
     switch (fdwReason) {